DESCRIPTION (Author Abstract): Amblyopia is the leading treatable cause of vision loss in childhood, with a prevalence of 2-5%. It is responsive to treatment early in life, but delayed treatment can result in life-long visual impairment. Unfortunately, health care practitioners are often unable to identify amblyopia risk factors, including strabismus, media opacities, and anisometropia, in patients under age 5, so that many cases of amblyopia go undetected and untreated. There is a need for a more effective method of detecting amblyopia risk factors. The fovea of the eye is surrounded by a distinctive pattern of birefrefringent fibers that change the polarization state of transmitted light. Our laboratory has developed a specialized form of retinal birefringence scanning (RBS), in which a small spot of polarized fight is scanned in a circle on the retina, and the returning fight measured for changes in polarization. We have demonstrated that RBS accurately (+/- 1 deg) detects foveal fixation in real time, in unrestrained subjects (including infants and children), making it possible to study patients with amblyopia and young children at risk for developing amblyopia. RBS has been characterized in only a small number of subjects, however, and little is known about individual variability. We have also developed binocular RBS (BRBS), which detects fixation of both eyes simultaneously and hence detects interocular alignment. The specific aims are to more fully characterize the RBS signal in normal and amblyopic subjects to enhance accuracy (hence sensitivity and specificity), and to screen normal and strabismic subjects using BRBS to identify amblyopia risk factors. BRBS may make it possible to screen infants and children automatically for the presence of amblyopia risk factors, including ocular misalignment, media opacity, and possibly refractive error, thereby facilitating early detection and treatment of this preventable form of blindness. The ability to screen for early, small deviations may help resolve conflicting findings on the efficacy of early amblyopia detection and treatment.